President Obama reportedly penned a secret letter to Iran's Supreme Leader Ayatollah Ali Khamenei last month discussing their shared interest in fighting the Islamic State -- a development one congressional source told Fox News "f***s up everything."

The Wall Street Journal reported Thursday that, according to people briefed on the letter, Obama wrote to Khamenei in the middle of last month and stressed that any cooperation on dealing with the Islamic State, or ISIS, was tied to Iran striking a deal over its nuclear program. The U.S., Iran and o...